I listened to these two sets of instrumental and solo piano works by the Swiss composer Jürg Frey in instalments over the space of a week, and his music’s capacity to linger – to evolve inside the inner ear once the sounds themselves have breathed their last – leaves a nourishing post-listening afterglow.

As a card-carrying member of the Wandelweiser composer collective, founded in 1992 to ponder those existentialist compositional questions raised by John Cage’s 4'33", Frey’s music privileges active listening over mere hearing. Ferne Farben (‘Distant Colours’, 2013) allows recordings of sounds captured from the environment licence to coexist alongside the blaring, turned-up quiet of glacially moving, hushed block chords scored for a characteristic Frey ensemble: flute, clarinet, organ, piano and string trio.

4'33" issued a reminder of the importance of environment – that music might just be the temporary frame we choose to place around the resonant harmonies and engine-room rhythms of real life, an idea that Frey has cunningly repurposed. Now the incidental hubbub, a genteel chorale of ambient breeze meets background babble punctuated by the occasional car horn, orchestrates and tints the instrumental…music? In the listening, it’s more as if Frey’s field recordings exhibit the harmonic tension and release we would normally ascribe to music as those instrumental sounds disperse into the surrounding atmosphere.

Ferne Farben – philosophically and sonically – represents Frey at his most explicitly Cageian, but each piece has a deep mystery of its own to communicate. On Philip Thomas’s disc of solo piano music, Extended Circular Music No 9 (2014 15) is a harmonic synecdoche. Frey’s harmonic sequences might evoke the over-ripe, ersatz Romanticism of a Broadway torch song or Barbra Streisand ballad, except that the transitionary connective passing-note tissue has been removed and suddenly these overly familiar harmonic phrases are governed by outside rules. Chromatic notes sigh, but the harmonic cushioning rarely falls where you anticipate. Then this cycle of harmonic dependency is abruptly broken: a single, high-register line ruminates obsessively around a narrow bandwidth of intervals, through which the underlying overtone series of the piano floats into view.

Overly candied Frey is invariably a disaster and Thomas’s deadpan distance feels absolutely right, a mood sustained throughout the standout Area of Three (2013). Reverberant cello drones set the scene as sustained interjections from piano, later clarinet, turn permutations like a harmonic Rubik’s cube. But then the delicate knit falls apart. Dressed up grace notes have nowhere to go. Piano chords are abandoned in silence. And the less material Frey utilises, the harder you listen.